The invention relates to a method for generating and storing digitalized density threshold values for the rastering of a half-tone picture original for the rastering of a half-tone picture original such as in the form of a color separation wherein at least one data storage the density threshold values of a segment of a raster are stored in dependence on a spot function as storage words.
The raster may be rotated opposite a scanning direction of a recording means which scans a recording carrier along scanning lines. Boundary lines of the segment of the raster extend in the scanning direction as well as in the direction rectangular thereto.
For the rastered recording of tonal value signals (picture signals) obtained by scanning of the half-tone picture original, it has been known to superimpose these tonal value signals with density threshold value signals (raster signals) of a raster rotated relative to a recording direction (German Patent Specification No. 1,901,101). The density threshold value signals, or raster signals, correspond to a density structure contents of a segment taken from the selected rotated raster, the boundary lines of which segment are arranged in the recording direction and in an advance direction orthogonal thereto. In the segment, the basic period of the structure of the rotated raster is contained once relative to each of the two-orthogonal directions. The segment in the scanning direction as well as at right angles thereto encompasses a plurality of raster points (spots) which are periodically arranged next to and above one another.
This segment may also be referred to as the multiple reference cell or the supercell. As a result of the periodicity of the segment of the rotated raster, the density threshold value signals (raster signals) can periodically be repeated without any difficulties in order to be superimposed by the tonal value signals of larger picture formats, or picture segments, respectively. The raster segment may, in addition, be resolved into so many partial lines extending in the picture recording direction that upon one picture line width a plurality of such partial lines will fall. In order to perform the described prior art process, use is made of data storages in which the density threshold values of the raster segment are digitally stored. Imagined visually, the density threshold values so stored may also be referred to as density fill or threshold fill, respectively. The density threshold values are so stored in the data storages so that the required raster information is fed by previous scanning of a pattern raster and quantizing and coding of the raster signals. The digital density threshold values thus contained in the data storages serve to be retransformed for superimposition with the tonal value signals scanned line by line wisely from the picture original into analog values and to be subsequently supplied to superimposition and threshold value units. When scanning an optical picture, round, oval or rectangular point shapes are typically employed which grow from the middle.
The problem on how in detail the density threshold values to be stored are suitably formed in order to so reproduce, in rastered fashion, a half-tone picture original so that the half-tone picture uniformly, or "quietly" acts over a partial surface which has the same tonal value, is not dealt with for the case that the threshold values, instead of by scanning of an optical picture, are formed fully in digital manner in accordance with a two-dimensional function. This function is also referred to as a spot function. For a fully digital generation of the density threshold values of the raster points of a multiple reference cell (supercell) in accordance with the state of the art known from practice, to start with the total number of storage words of the multiple reference cell is determined. The determination of the total number may be conducted in dependence on the raster angle, the raster width and the resolution of the system. A sorted sequence of the storage words of the multiple reference cell which depends on the spot function is then formed. The storage words of the multiple reference cell will then be assigned density threshold values in linear dependency on their position in the sorted sequence.
In fact, the individual raster points (spots) of a multiple reference cell (supercell) are somewhat different insofar as the number of the storage words assigned to one raster point each varies and does not, as a rule, correspond to the nominal value resulting from the total number of the storage words of the multiple reference cell and the number of the raster points, or subcells, respectively. This leads to the consequence that in neighboring raster points, more pixels or less are set if, depending on a predetermined gray tone, a particular fraction of all the pixels assigned to the multiple reference cell is to be blackened. Thereby, the observer of the reproduced half-tone picture original receives the impression of differently large black points on a white background and, in any case if the tonal value is distinctly less than 50 percent. If on the other hand, a darker gray tone, which is distinctly over 50 percent, for instance 70 percent, is reproduced by using a multiple reference cell, for similar reasons as given above, for low gray values the impression of differently large bright spots on a black background will come up. In both cases, the reproduced half-tone picture is not quiet.